He isn't playing as well as last year but i think your making far to much out of this, you are also downplaying the speed and skill of the arsenal back line who concentrated more on keeping ronaldo out of the game than any other united player.
It is obvious ronaldo will find it harder to start with this season as he was so good last year teams will mark him more and try to stop him playing, this is one reason why tevez and ronney are doing so well, people are worried about ronaldo so they are giving rooney and tevez more space.